1. Field of the Invention
The present invention relates to a GaN single crystal substrate suitable for manufacture of a GaN-based semiconductor device having characteristics with small variations, specifically to a GaN single crystal substrate having a large area and a main surface whose plane orientation is other than (0001) and (000-1) (namely other than {0001}), and having at least one of a substantially uniform distribution of the carrier concentration in the main surface, a substantially uniform distribution of the dislocation density in the main surface, and a small photoelasticity distortion in the main surface that is a distortion measured by photo elasticity, and relates to a method of manufacturing the GaN single crystal substrate. The invention also relates to a GaN-based semiconductor device including such a GaN single crystal substrate and at least one GaN-based semiconductor layer formed on the main surface of the substrate, and a method of manufacturing the GaN-based semiconductor device.
2. Description of the Background Art
Group III nitride crystals used suitably for light emitting devices, electronic devices, semiconductor sensors and the like are usually manufactured by means of vapor phase methods such as HVPE (hydride vapor phase epitaxy) and MOCVD (metal organic chemical vapor deposition), and liquid phase methods such as flux method, to grow a crystal on a main surface of a sapphire substrate where the main surface is a (0001) plane, or a main surface of a GaAs substrate where the main surface is a (111) A plane. Therefore, a group III nitride crystal as usually obtained has a main surface whose plane orientation is {0001}.
Regarding a light emitting device including a substrate of a group III nitride crystal having a main surface whose plane orientation is {0001}, and a light emitting layer of an MQW (multiple quantum well) structure on the main surface, the polarity in the <0001> direction of the group III nitride crystal causes spontaneous polarization in the light emitting layer, resulting in a large blue shift of light emission and a deteriorated luminous efficacy. A group III nitride crystal having a main surface whose plane orientation is other than {0001} is therefore needed.
In order to meet such a need, Japanese Patent Laying-Open No. 2008-143772 (Patent Document 1) discloses a method of manufacturing a group III nitride crystal having a main surface whose plane orientation has an off angle of 5° or less with respect to one of the plane orientations {1-10X} (where X is an integer of not less than 0), {11-2Y} (where Y is an integer of not less than 0}, and {HK−(H+K)0} (where H and K are each an integer other than 0), specifically a main surface whose plane orientation is one of {1-100}, {11-20}, {1-102}, {11-22}, {12-30}, and {23-50}.
Even if the manufacturing method disclosed in Japanese Patent Laying-Open No. 2008-143772 (Patent Document 1) is used, the group III nitride crystal grown on the main surface whose plane orientation is {1-100}, {11-20} or {23-50} is partially polycrystallized, resulting in a problem that a single crystal substrate of a large area is difficult to obtain. Further, at a crystal growth surface of the group III nitride crystal grown on the main surface having the plane orientation {1-102} or {11-22}, a facet with the plane orientation {0001} and a facet with a plane orientation other than {0001} are generated.
Here, the efficiency in taking in impurities from a facet of the plane orientation {0001} is considerably different from that from a facet of a plane orientation other than {0001}. A resultant problem is therefore a large variation of the carrier concentration and a large variation of the specific resistance in the main surface of the group III nitride crystal having been grown, and accordingly large variations of characteristics of a semiconductor device for which such a substrate is used.
Further, in a growth portion where a facet of the plane orientation of {0001} is a crystal growth surface, dislocation propagates perpendicularly to the {0001} plane (namely the <0001> direction). In a growth portion where a facet of a plane orientation other than {0001} is a crystal growth surface, dislocation propagates in parallel with the {0001} plane. A resultant problem is therefore a large variation of the dislocation density in the main surface of a grown group III nitride crystal, and thus large variations of characteristics of a semiconductor device using such a substrate.
Furthermore, because of a large variation of the dislocation density in the main surface of the grown group III nitride crystal, a group III nitride crystal substrate made from such a group III nitride crystal has a main surface in which a microscopic variation of distortion is generated, resulting in a problem that a large local distortion is generated.
Japanese Patent Laying-Open No. 2005-101475 (Patent Document 2) discloses a group III-V nitride based semiconductor substrate (specifically free-standing GaN substrate) having a substantially uniform distribution of the carrier concentration, and a method of manufacturing the semiconductor substrate. Although Japanese Patent Laying-Open No. 2005-101475 (Patent Document 2) discloses, regarding a group III-V nitride based semiconductor substrate having a main surface whose plane orientation is (0001), that crystal growth is made flat to provide a substantially uniform distribution of the carrier concentration, Patent Document 2 does not disclose or suggest that the distribution of the carrier concentration is made substantially flat for a group III-V nitride based semiconductor substrate having a main surface whose plane orientation is other than {0001}.
Further, Japanese Patent Laying-Open No. 2006-052102 (Patent Document 3) discloses a group III-V nitride based semiconductor substrate (specifically free-standing GaN substrate) having a substantially uniform distribution of the dislocation density, and a method of manufacturing the semiconductor substrate. Japanese Patent Laying-Open No. 2006-052102 (Patent Document 3) discloses, regarding a group III-V nitride based semiconductor substrate having a main surface whose plane orientation is (0001), that a crystal is grown while concaves are generated on a growth interface, the growth interface is flattened, and the crystal is further grown on the flattened growth interface, so that the dislocation density distribution is made substantially uniform. Patent Document 3, however, does not disclose or suggest that the dislocation density distribution is made substantially uniform for a group III-V nitride based semiconductor substrate having a main surface whose plane orientation is other than {0001}
Japanese Patent Laying-Open No. 2002-299741 (Patent Document 4) discloses a GaN single crystal substrate having a photoelasticity distortion value of not more than 5×10−5 that is measured by photoelasticity in a main surface of the substrate. Japanese Patent Laying-Open No. 2002-299741 (Patent Document 4) merely discloses, regarding a GaN single crystal substrate having a main surface whose plane orientation is (0001), that a photoelasticity distortion value in the main surface is not more than 5×10−5, and does not disclose or suggest, regarding a GaN single crystal substrate having a main surface whose plane orientation is other than (0001) and (000-1), the maximum value and a variation of the photoelasticity distortion value in the main surface.